The present invention relates to an infrared phosphor which is excited by irradiation with infrared rays to emit light of infrared wavelength region, and to a material having latent images and an optical reading system which use said infrared phosphor.
Recently, in various industries such as the distribution industry, control of goods by means of bar codes is extensively conducted and furthermore, bar codes are printed on various prepaid cards and passing cards and these bar codes are read by optical reading apparatuses such as scanners to perform the desired actions. Moreover, various attempts have been made to apply forgery preventive means to credit cards and prepaid cards or to detect forged cards. According to one of them, the marks such as bar codes are printed with an ink containing a phosphor by offset printing or by using an ink ribbon to form latent image marks, the latent image marks are irradiated with a semiconductor laser beam to excite the phosphor and the light emitted from the phosphor is received to read the bar code information by an optical reading apparatus.
The phosphors used in the above-mentioned method are those which are described in U.S. Pat No. 4,202,491.
However, the conventional phosphors have a large particle size of larger than 5 .mu.m and cannot be used as they are in the inks used for offset printing or ink ribbons and they must be ground. During this grinding step, crystallinity and composition of the phosphors are damaged to cause considerable reduction in luminous intensity. Another problem is reduction in productivity due to the increase in the number of steps. For this reason, the systems in which the above phosphors are used have not yet been able to be put to practical use.